A large body of data has been accumulated that establishes a role for a family of receptors, the selectins (hereinafter LEC-CAMS), in certain diseases including cancer, autoimmunity, and in the inflammatory response. The three known members of this family, L-Selectin (LECAM-1, LAM-1, gp90MEL), E-Selectin (LECAM-2, ELAM-1) and P-Selectin (LECAM-3, GMP-140, PADGEM), each contain a domain with homology to the calcium-dependent lectin (C-lectins), an EGF-like domain, and several complement-binding protein-like domains (Bevilacqua et al, Science (1989) 243:1160-1165; Johnston et al, Cell (1989) 56:1033-1044; Lasky et al, Cell (1989) 56:1045-1055; Tedder et al, J. Exp. Med. (1989) 170:123-133). It has been proposed that the selectins bind to particular ligands and that this accounts for their biological activity. Thus, drugs that interfere with or prevent binding of the ligands to the selectins will be useful medicaments for treating a variety of diseases.
For instance, adhesion of circulating neutrophils to stimulated vascular endothelium is a primary event of the inflammatory response. P-selectin has been shown to be centrally involved, particularly as related to acute lung injury. Mulligan et al, J. Clin. Invest. (1991) 90: 1600, report strong protective effects using anti-P-selectin antibody in a rodent lung injury model.
Of the three selecting, ELAM-1 is particularly interesting because of its transient expression on endothelial cells in response to IL-1 or TNF (Bevilacqua et al, supra). The time course of this induced expression (2-8 h) suggests a role for this receptor in initial neutrophil extravasation in response to infection and injury. Indeed, Gunel et al, J. Clin. Invest. (1991) 88:1407, have shown that antibody to ELAM-1 blocks the influx of neutrophils in a primate model of asthma and thus is beneficial for preventing airway obstruction resulting from the inflammatory response.
Lowe et al, Cell (1990) 63:475, demonstrated a positive correlation between ELAM-1-dependent adhesion of HL-60 cell variants and transfected cell lines, with their expression of the sialyl Lewis x (sLe.sup.x) oligosaccharide, Neu NAc .alpha.2-3Gal-.beta.1-4(Fuc .alpha.1-3)-GlcNAc. By transfecting cells with plasmids containing an .alpha.(1,3/1,4) fucosyltransferase, they were able to convert non-myeloid COS or CHO lines into sLe.sup.x -positive cells that bind in an ELAM-1 dependent manner. Walz et al, Science 250 (4984):1132 (1990) were able to inhibit the binding of a ELAM-1-1gG chimera to HL-60 cells with a monoclonal antibody directed against sLe.sup.x or by glycoproteins with the sLe.sup.x structure, but could not demonstrate inhibition with CD65 or CD15 antibodies. Both groups concluded that the sLe.sup.x structure is the ligand for ELAM-1.
ELAM-1 is a glycoprotein that is found on the surface of endothelial cells, the cells that line the interior wall of capillaries. E-selectin recognises and binds to sLe.sup.x, which is present on the surface of certain white blood cells. E-selectin helps white blood cells recognise and adhere to the capillary wall in areas where the tissue surrounding the capillary has been infected or damaged. P-selectin is expressed on inflamed endothelium and platelets, and has much structural similarity to E-selectin and can also recognise sLe.sup.x.
When a tissue has been invaded by a microorganism or has been damaged, white blood cells, also called leukocytes, play a major role in the inflammatory response. One of the most important aspects of the inflammatory response involves the cell adhesion event. Generally, white blood cells are found circulating through the bloodstream. However, when a tissue is infected or becomes damaged, the white blood cells must be able to recognise the invaded or damaged tissue and be able to bind to the wall of the capillary near the affected tissue and diffuse through the capillary into the affected tissue. E-selectin helps two particular types of white blood cells recognise the affected sites and bind to the capillary wall so that these white blood cells may diffuse into the affected tissue.
There are three main types of white blood cells: granulocytes, monocytes and lymphocytes. Of these categories, E-selectin recognises sLe.sup.x presented as a glycoprotein or glycolipid on the surface of monocytes and neutrophils. Neutrophils are a subclass of granulocytes that phagocytise and destroy small organisms, especially bacteria. Monocytes, after leaving the bloodstream through the wall of a capillary, mature into macrophages that phagocytise and digest invading microorganisms, foreign bodies and senescent cells.
Monocytes and neutrophils are able to recognise the site where tissue has been damaged by binding to E-selectin, which is produced on the surface of the endothelial cells lining capillaries when the tissue surrounding a capillary has been infected or damaged. Typically, the production of E- and P-selectins is increased when the tissue adjacent to a capillary is infected. P-selectin is present constitutively in storage granules from which it can be rapidly mobilized to the cell surface after the endothelium has been activated. In contrast, E-selectin requires de novo RNA and protein synthesis, peak expression does not occur until about 4-6 hours after activation, and expression declines to basal levels after about 24-48 hours. White blood cells recognise affected areas because sLe.sup.x moieties present on the surface of the white blood cells bind to E- and P-selectin. This binding slows the flow of white blood cells through the bloodstream, since it mediates the rolling of leukocytes along the activated endothelium prior to integrin-mediated attachment and migration, and helps to localise white blood cells in areas of injury or infection.
While white blood cell migration to the site of injury helps fight infection and destroy foreign material, in many instances this migration can get out of control, with white blood cells flooding to the scene, causing widespread tissue damage. Compounds capable of blocking this process, therefore, may be beneficial as therapeutic agents. Thus, it would be useful to develop inhibitors that would prevent the binding of white blood cells to E- or P-selectin. For example, some of the diseases that might be treated by the inhibition of selectin binding to sLe.sup.x include, but are not limited to, ARDS, Crohn's Disease, septic shock, traumatic shock, multi-organ failure, autoimmune diseases, asthma, inflammatory bowel disease, psoriasis, rheumatoid arthritis and reperfusion injury that occurs following heart attacks, strokes and organ transplants. In addition to being found on some white blood cells, sLe.sup.a, a closely related regiochemical isomer of sLe.sup.x, is found on various cancer cells, including lung and colon cancer cells. It has been suggested that cell adhesion involving sLe.sup.a may be involved in the metastasis of certain cancers and that inhibitors of sLe.sup.a binding may be useful in the treatment of some forms of cancer.
As alluded to above, the ligands for the selectins generally consist of carbohydrate structures containing at least sialic acid, fucose and lactose. Lactose consists of galactose and glucose. Considering the obvious medical importance of selectin ligands, significant effort has been, and continues to be expended to identify the critical physical/chemical parameters associated with selectin ligands that enhance, or that are required for their activity. The majority of this work has centred around modifications to the natural ligands sLe.sup.x, sLe.sup.a and variants thereof, as outlined in Ramphal et al, J. Med. Chem. (1996) 39:1357-1360. A range of carbohydrate mimics that retain the key functional units of the natural ligand have also been reported, by Ohmoto et al, J. Med. Chem. (1996) 39:1339-1343. More recently, some non-carbohydrate selectin ligands have been disclosed; see, for example, WO-A-9529681.
WO-A-9619231 discloses anti-inflammatory oligosaccharides of the type sugar-sugar-X-aglycone, wherein X is O, S, N or C.
Singh et al, J. Nat. Prod. (1990) 53(5): 1187-92, disclose the isolation of a weakly cytostatic natural product named aceratioside. It comprises a sugar moiety attached to the aromatic ring of a 1,2,3,4-tetrahydronaphthalene nucleus.